1. Field of the Invention
The present invention relates to a rotor for an automotive alternator, which has a Lundell-type field core, for mounting on an automobile engine, and in particular, relates to a winding configuration for winding onto the Lundell-type field core.
2. Description of the Related Art
FIG. 6 is a cross-section of a conventional rotor for an automotive alternator, FIG. 7 is a cross-section of part of the rotor shown in FIG. 6, and FIG. 8 is a perspective view of a bobbin used in the rotor shown in FIG. 6.
In FIGS. 6 to 8, a rotor 1 comprises a rotating shaft 11 rotatably supported by a bracket (not shown), a pair of Lundell-type field cores 12a, 12b secured to the rotating shaft 11, a pair of fans 13a, 13b secured to both axial ends of the field cores 12a, 12b, slip rings 14 secured to one end of the rotating shaft 11, and a field winding 15 wound onto the field cores 12a, 12b.
The field cores 12a, 12b are made of iron, comprise cylindrical base portions 121a, 121b fitted over and secured to the rotating shaft 11 and claw-shaped magnetic poles 122a, 122b plurally projecting from the outer circumferential edges of the base portions 121a, 121b, and are secured to the rotating shaft 11 facing each other such that the end surfaces of the base portions 121a, 121b are in close contact with each other and the claw-shaped magnetic poles 122a, 122b intermesh alternately. The field winding 15 is a copper wire with a circular cross-section and is wound a predetermined number of times onto a bobbin 16 fitted over the outer circumferences of the base portions 121a, 121b. A magnetic flux is generated when an electric current is supplied to the field winding 15 by means of the slip rings 14 and magnetic poles are formed in the field cores 12a, 12b by the magnetic flux.
The bobbin 16 is made of resin, and as shown in FIG. 8, comprises a cylindrical portion 16a and a pair of first and second annular flange portions 16b projecting perpendicularly from both ends of the cylindrical portion 16a. A recessed groove 161 with a U-shaped cross-section for housing a lead wire 15a at the start of the winding is disposed radially in the inner wall of the first flange portion 16b so as to extend from the outer circumferential side thereof to the cylindrical portion 16a. An anchor portion 16c is disposed on an outer circumferential portion of the first flange portion 16b in close proximity to the outer circumferential end of the recessed groove 161.
Inner circumferential tape 17a for protecting the winding is wound onto the cylindrical portion 16a of the bobbin 16. Outer circumferential tape 17c for protecting the winding is also wound onto the outer circumference of the field winding 15 wound onto the bobbin 16. In addition, side tape 17b is disposed between the lead portion of the field winding 15 and the multi-layered portion of the field winding 15.
The construction of the field winding 15 will now be explained.
First, the inner circumferential tape 17a is wound onto the cylindrical portion 16a of the bobbin 16. Then, the starting portion of the field winding 15 is wound around the anchor portion 16c, inserted into the recessed groove 161, and drawn from the lower end (inner circumferential end) of the recessed groove 161 along the inner wall of the first flange portion 16b onto the cylindrical portion 16a. At this point, the side tape 17b is pasted onto the inner wall of the first flange portion 16b so as to cover the lead wire 15a at the start of the field winding 15 which is housed in the recessed groove 161. Then, the field winding 15 drawn out onto the cylindrical portion 16a is lined up in rows in the axial direction as it is wound onto the cylindrical portion 16a from the first flange portion 16b to the second flange portion 16b. Then, when the first layer of the winding is finished, the field winding 15 is lined up in rows in the axial direction as it is wound onto the cylindrical portion 16a from the second flange portion 16b to the first flange portion 16b. In this way, the field winding 15 is wound up layer by layer in order from the bottom of the cylindrical portion 16a, and when a predetermined number of layers have been wound, the outer circumferential tape 17c is wound onto the outermost circumferential portion. In addition, the multi-layered portion of the field winding 15 is saturated with varnish.
In the rotor 1 constructed in this manner, centrifugal force acts constantly on the field winding 15 during power generation, and even slight gaps and looseness are gradually enlarged, leading to disarray in the winding. Thus, in order to achieve winding without gaps or looseness, it is usual to apply tension to the wire as it is wound onto the bobbin 16, and the starting configuration, in which the field winding 15 is wound onto the anchor portion 16c of the bobbin 16 and further housed in the recessed groove 161 disposed in the inner wall of the flange portion 16b of the bobbin 16, has also been adopted for this purpose.
In a conventional rotor for an automotive alternator constructed in this manner, the field winding 15, which has a circular cross-section, is lined up in rows in the axial direction as it is wound onto the cylindrical portion 16a. Thus, when the second layer is wound and lined up on top of the circumferentially innermost first layer after the first layer has been completely wound and lined up in rows in the axial direction, the center of the wire in the second layer is displaced by one radius in the axial direction, as shown in FIG. 7.
In a conventional winding configuration of this kind, portions of the winding are in contact with other portions at points, and since many layers are wound, the winding pressure and the weight of winding in the outer circumference act on the winding in the inner circumference, and there is a risk that the wire will be deformed or displaced in the axial direction.
Also, if the center of the wire is displaced by more than one radius or less than one radius in the axial direction, portions of the winding ride on adjacent portions and large irregularities arise, which soon leads to disarray as shown in FIG. 9. Such irregularities accumulate and increase towards the outer circumference, making alignment of the winding in the outer circumference difficult.
Thus, one problem is that the configuration of the multi-layered portion constructed by winding the field winding 15 onto the cylindrical portion 16a in many layers is effectively eccentric, and the eccentric portions and non-circular portions are gradually enlarged by the centrifugal force acting during high-speed rotation, giving rise to frequent disarray and increasing vibrations in the multi-layered portion, which leads to bending of the rotating shaft 11 or disconnection of the winding connections.
Also, in the conventional rotor, the field winding 15 is housed in the recessed groove 161 disposed in the inner wall of the flange portion 16b of the bobbin 16 so that tension can be applied to the winding as it is wound onto the bobbin 16. However, the shape of the recessed groove 161 is determined by the circular cross-section of the field winding 15 and, as shown in FIG. 10, an axially thin-walled portion is formed in the bobbin 16, most of which is already formed to a thickness of 1 mm or less. As a result, another problem is that cracks form in the thin-walled portion, giving rise to malfunction due to interference between the winding and the core.
In order to avoid this, projection of the outer circumferential portion of the recessed groove 161 outwards has been considered, as shown in FIG. 11, to ensure that the outer circumferential portion of the recessed groove 161 is strong. However, in that case, it is necessary to provide a recessed portion in the portion of the field core 12a corresponding to the projecting portion on the outer circumferential portion of the recessed groove 161, which influences the quality and performance of the field core 12a as a rotating body requiring balance.